Draft 1: Initial Situation

Even if there is wind pressure on the exit portal 1 , the piston effect of the vehicles produces an actual airflow 2 in the direction of traffic 3. 

Draft 2: Accident

When fire breaks out due to an accident 1 , the smoke produced first flows towards the exit portal 2 . Vehicles beyond the site of the accident leave the tunnel 3 , those before it come to a halt 4.

Draft 3: Inversion of Airflow

When all vehicles have either left the tunnel or are stalled before the place of the fire 1 , the piston effect of the moving vehicles ceases. This results in an inversion of the airflow in the tunnel, so that the rising smoke begins to stream towards the stalled vehicles 2 .